clk: add function documentation for clk_hw_round_rate()

Information about usage and prerequisites for this API.

Signed-off-by: Sarang Mairal <sarangmairal@gmail.com>
Link: https://lore.kernel.org/r/20200611021941.786-2-sarangmairal@gmail.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>